Bangkok: A Thai gardener behind a $20 million gem heist from a Saudi palace that has long soured relations between the two countries became a monk Thursday in hope of redeeming his karma.

Kriangkrai Techamong stole the precious gems from the palace of a Saudi prince where he worked in 1989, triggering a feud between Thailand and Saudi Arabia dubbed the "Blue Diamond Affair" that has yet to be resolved.

Thai police later returned some of the jewels but Saudi officials claimed most were counterfeits while the whereabouts of the most precious gem — a rare 50-carat blue diamond — remains unknown.

Representational image. Reuters

On Thursday Kriangkrai told local media his life has been haunted by the theft that unleashed an "avalanche" of suffering on his family.

"I am confident that all my misfortunes are the result of a curse from the (blue) Saudi diamond I stole, so I've decided to enter the monkhood for the rest of my life to redeem my bad karma," he told Thai Rath newspaper.

Local TV channels showed the middle-aged man receive alms from templegoers as he marched in an ordination ceremony with a shaved head and white robes in northern Lampang province.

Channel 7 reported that he had been blessed with a new monk name that translates to "He Who Has Diamond Knowledge".

Kriangkrai was jailed for five years soon after the theft, but managed to sell most of the gems before his arrest.

Saudi Arabia has long accused Thai police of bungling its investigation, with widespread allegations at the time that the stolen items were snapped up by senior officers.

Riyadh sent a businessman to conduct his own investigation, but he disappeared in Bangkok days after three Saudi diplomats were shot dead, execution style, in the city.

In 2014 a case was dropped against five men, including a senior Thai policeman, for alleged involvement in the businessman's murder over lack of evidence. The decision came after a last-minute change in judge.

Saudi Arabia has not sent an ambassador to Thailand for decades and restricts travel between the two countries because of the unresolved theft and murders.